An image forming apparatus provided with a developing device for developing an electrostatic image formed on an image bearing member, into a toner image, using the developer (two component developer) containing toner and carrier is widely used. An image forming apparatus comprising an exchangeable developer container containing a reservoir developer including toner in which the reservoir developer is exchanged to supply the developer is widely used. Ordinarily, the developing device is provided with a sensor for detecting a toner content (toner weight ratio in the developer), and an amount of the developer response to the sensor output and the toner consumption amount for each image forming operation is supplied into the developing device in accordance with image forming operation.
In the image forming apparatus provided with the exchangeable developer container, it is not the fact that immediately after the start of the supply of the reservoir developer by the developer supply portion after the exchange of the developer container, the supplied developer flows into the developing device. Not less than ten sec is required for the developer supplied from the developer container to reach and to be stirred and mixed and to recover the toner content.
For this reason, with the image forming apparatus provided with the exchangeable developer container, it is ordinary that when the developer container is exchanged, the image forming operation is temporarily interrupted, and the developer supply portion is operated for not less than ten sec, and then the image forming operation is permitted (Japanese Laid-open Patent Application 2005-62848).
However, in the case that the developer supply portion is automatically operated after the exchange of the developer container, the image forming operation is not possible for that time period, with the result of down time of the image forming apparatus. Particularly in the case of a full-color image forming apparatus comprising a plurality of developing containers, the frequency of the occurrences of such down time is multiplied by the number of the developer containers.
In view of this, Japanese Laid-open Patent Application 2007-65325 discloses an image forming apparatus of one component developing type (magnetic toner type), in which the image forming operation is enabled to start immediately after the exchange of the developer container. The developer supply portion is provided with a multi-level toner level sensor, and the image forming operation is stopped only when the toner level becomes lower than the lowest toner level, and the toner supply from the developer supply portion is continued.
In the two component developing type developing device, the developer amount in the developing device hardly changes whatever amount of the toner is consumed, and therefore, the method using the toner level sensor disclosed in Japanese Laid-open Patent Application 2007-65325 cannot be employed. Then, it has been proposed that the image forming operation is permitted immediately after the exchange of the developer container, and thereafter, when the toner content in the developing device becomes lower than a first reference value which is set to be lower than the normal level, the image forming operation is prevented, and the toner supply from the developer supply portion is continued.
However, the output of the toner density sensor significantly changes depending on the temperature and humidity, the toner charge amount and the fluidity state of the developer, and therefore, the discrimination based on the output of the toner density sensor may lead to failure of the necessary prevention of the image forming operation. If the first reference value is raised in a attempt to assure the prevention of the image forming operation in order to avoid excessive decrease of the toner content, the frequency of unnecessary preventions of the image forming operation increases, with the result of increase of the down time of the image forming apparatus.